Supporting Goals-Based Learning with STEM Outreach (Figure 1)

Research Summary: Ideas Students Have about Light

Students may think that light helps us see a shadow that is always there, or that light causes an object to produce, cast, or push a shadow out of an object

Shadows are formed when an object blocks the path of traveling light. In contrast, students may speak of shadows as if they were the presence of something that has material characteristics (as opposed to the absence of light), or may describe shadows as “images,” “pictures,” or “reflections” that are the same shape as and look like the object (13, 16). As a result, students may not accept that a shadow is formed behind an object if a light source is placed directly in front of the object. If students count shadows as dark images that look like the object, then an amorphous black spot does not count as a shadow (17).

Students may have alternative explanations for how shadows are formed:

  1. They may think that shadows are always present but that light is needed to see them (“It’s there at night. You just can’t see it”) or

  2. They may think that light causes an object to produce/cast a shadow or that light pushes the shadow out. Students describe shadows as shot out by objects when triggered by light (“When light hits an object, the object reflects the shadow”), or even as actively pushed by light (“Light pushes the shadow like a wave pushes a ball in water”) (16).

Use of the terms “reflection” and “reflect” in students’ descriptions and explanations of shadows does not necessarily mean that students think that shadows form by light bouncing off objects. Students often use the term “reflection” loosely to describe the similarity of form between the object and the shadow. Similarly, students often appear to use the verb “reflect” synonymously with “hit,” “move,” and “project” without any reference to light bouncing off or changing direction. Students’ statements such as “the sun reflects your shadow on the ground,” “when light hits an object, the object reflects the shadow,” or even “the light reflects off you and bounces down there to make the shadow” need to be probed further to ascertain whether the idea “shadows are formed by light hitting the object and changing direction” is underlying these responses. Students may correctly predict the relative location of the light source, object, and shadow but account for the formation of shadows using one of the alternative explanations. For example, some students predict the correct location of a shadow but believe that light pushes the shadow out of the object. Furthermore, some students refer to objects blocking light and causing shadows, but their use of the term “blocking” indicates a belief that when light is blocked by an object, a shadow is forced out (17).

Likely Sources of Student Ideas

The shadows we see under usual circumstances when we stand on the pavement in the sun are caused by objects that are far from the light source and close to the screen (the surface on which the shadow forms). In these cases, the shape of the shadow is indeed the same as the shape of the object. Probably for this reason we ascribe the shadow to the object; we speak of “its” shadow or “our” shadow in the possessive. Furthermore, we say the object casts its shadow as if it were throwing out something previously held within it. Children’s literature reinforces this view: Peter Pan’s shadow has Peter Pan’s shape, it moves around with him constantly, and is material enough that Wendy can sew it back on when it accidentally becomes detached (16).

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